1. Field of the Invention
The present invention relates to a color-ink set of plural color inks including a black ink, for use in recording of a color image on a recording medium in combination with the plural color inks. The present invention also relates to an ink-jet recording method and an ink-jet apparatus employing the ink set. The present invention particularly relates to a color ink set, and an ink-jet recording method and an ink-jet apparatus employing the ink set for forming an image on plain paper with sufficient image density, sufficient sharpness, and high quality of the image with improvement in the water resistance and lightfastness of the print.
2. Related Background Art
In ink-jet recording, for formation of a black image, various black inks employing a pigment are disclosed which can achieve formation of a black image on plain paper with high image density, high print quality, and high durability. Furthermore, ink sets are disclosed which does not cause bleeding at the boundary between a black print area and a color print area in the printed portion. This phenomenon is hereinafter referred to simply as xe2x80x9cbleedingxe2x80x9d. Ink-jet recording methods and apparatuses therefor are also disclosed.
As to the ink, Japanese Patent Application Laid-Open No. 3-210373 discloses a black ink comprising an acidic carbon black and an alkali-soluble polymer. Japanese Patent Application Laid-Open No. 3-134073 discloses an ink-jet recording ink which gives a dispersion excellent in storage stability and having excellent ink ejection properties in ink-jet recording.
As to the ink set, Japanese Patent Application Laid-Open No. 6-57192 discloses an ink set for ink-jet recording which comprises a black ink containing at least one anionic dye, and a yellow ink containing at least one cationic dye and one multivalent precipitant, and can prevent bleeding. Japanese Patent Application Laid-Open No. 7-145336 discloses an ink set which causes less bleeding in multi-color printing by employing an anionic ink and a cationic ink, at least one of the inks containing a polymer having ionic properties of the ink, and by bringing into contact the anionic ink and the cationic ink in the presence of the polymer.
As to the ink-jet printing method and the ink set, Japanese Patent Application Laid-Open No. 7-1837 discloses an ink-jet recording method causing less bleeding by employing a black ink containing a coloring agent comprising a pigment and a dispersant which is insolubilized at a specified pH, and a color ink having a pH for insolubilizing the dispersant, and also discloses an ink set therefor. Japanese Patent Application Laid-Open No. 10-183046 discloses an ink-jet printing method employing a black ink containing a coloring material of a carbon black having a cationic group on the surface, and a color ink containing an anionic dye as the colorant, and also discloses an ink set therefor.
For the ink disclosed in the aforementioned Japanese Patent Application Laid-Open No. 3-210373 which comprises an acidic carbon black and an alkali-soluble polymer, the carbon black is dispersed by the alkali-soluble polymer. However, this dispersion has a high viscosity, which causes problems in the storage stability. The ink disclosed in the aforementioned Japanese Patent Application Laid-Open No. 3-134073 can gives a dispersion which has sufficient storage stability as well as excellent ejecting properties for a bubble jet recording apparatus. However, this ink is liable to be insufficient in print density.
The ink set disclosed in the aforementioned Japanese Patent Application Laid-Open No. 6-57192, which comprises a black ink containing an anionic dye as a colorant and a yellow ink containing a cationic dye and a multivalent precipitant, cannot satisfy all of the print density, print quality, image durability of the black ink simultaneously, and does not give the desired color developability of the yellow ink, although the ink set reduces the bleeding significantly.
The ink set disclosed in the aforementioned Japanese Patent Application Laid-Open No. 7-145336, which can reduce the bleeding by employing an anionic ink and a cationic ink and bringing into contact the anionic ink and the cationic ink in the presence of the polymer in multi-color printing, but can cause adverse effects on ink reliability depending on the type of the polymer. In particular, when an ink ejection from one nozzle is suspended for a certain time period (e.g., one minute) during printing, a subsequent ink ejection through the nozzle sometimes becomes unstable to cause irregularity in the print. Such unstable subsequent ink ejection is called xe2x80x9cfirst droplet ejection failurexe2x80x9d, and the stability of intermittent ejection is called xe2x80x9cfirst ejection stabilityxe2x80x9d in the specification of the present invention.
For the ink set disclosed in the aforementioned Japanese Patent Application Laid-Open No. 7-1837 which employs a black ink containing a coloring agent comprising a pigment and a dispersant insoluble at a specific pH, and a color ink having a pH for insolubilizing the dispersant. This ink set is limited in selection of the dye owing to the required acidity (pH) of the color ink in consideration of the other properties required for the color ink. This Laid-Open publication does not describe the polarity of the coloring material.
The ink set disclosed in the aforementioned Japanese Patent Application Laid-Open No. 10-183046, which employs a black ink containing a coloring material of a carbon black having a cationic group on the surface and a color ink containing an anionic dye as the colorant, does not sufficiently reduce the bleeding, although this ink set is satisfactory in the print density, print quality, image durability, and other properties.
An object of the present invention is to provide an ink set which forms an image satisfying the required properties of the black ink such as print quality, and image durability with more effective alleviation of the bleeding at the boundary between a black area and a non-black color area of an image formed on a recording medium.
Another object of the present invention is to provide an ink-jet recording method which forms a recorded image of high quality with effective reduction of the bleeding at the boundary between a black area and a non-black color area of an image formed on a recording medium.
Still another object of the present invention is to provide an ink-jet recording apparatus which forms a recorded image of high quality with effective reduction of the bleeding at the boundary between a black area and a non-black color area of an image formed on a recording medium.
A further object of the present invention is to provide a recording unit which can be used for the above ink-jet recording apparatus.
A still further object of the present invention is to provide an ink cartridge for forming an image with high quality with less bleeding.
A still further object of the present invention is to provide a method for reliably reducing the bleeding at the boundary between a black image area and a colored image area.
In one embodiment of the present invention, there is provided an ink set for recording a color image with inks of two or more colors comprising at least one black ink and one color ink on a recording medium, the black ink containing a self-dispersible carbon black having at least one cationic group bonded directly or through another atomic group to the surface thereof, and the color ink containing an anionic substance and having a buffering capability against a pH change.
In another embodiment of the present invention, there is provided an ink-jet recording method comprising the steps of:
applying a black ink containing a self-dispersible carbon black having a cationic group bonded directly or through another atomic group to the surface thereof onto a recording medium by ejecting the black ink through an orifice in response to a recording signal; and applying a color ink containing an anionic substance and having a buffering capability against a pH change onto the recording medium by ejecting the color ink through an orifice in response to another recording signal.
In still another embodiment of the present invention, there is provided a recording unit comprising a black ink holder holding a black ink containing a self-dispersible carbon black having a cationic group bonded directly or through another atomic group to the surface thereof; a color ink holder for holding a color ink containing an anionic substance and having a buffering capability against a pH change; and a head for ejecting the black ink and the color ink respectively.
In still another embodiment of the present invention, there is provided an ink cartridge comprising a black ink holder holding a black ink containing a self-dispersible carbon black having a cationic group bonded directly or through another atomic group to the surface thereof; and a color ink holder holding a color ink containing an anionic substance and having a buffering capability against a pH change.
In still another embodiment of the present invention, there is provided an ink-jet recording apparatus comprising a black ink holder holding a black ink containing a self-dispersible carbon black having a cationic group bonded directly or through another atomic group to the surface thereof; a color ink holder holding a color ink containing an anionic substance and having a buffering capability against a pH change; and a head for ejecting the black ink and the color ink respectively.
In a further embodiment of the present invention, there is provided a method for alleviating bleeding at a boundary between a black color image and a color image each formed on a recording medium by ink-jet method comprising the steps of:
forming the black color image by using a black ink containing a self-dispersible carbon black having a cationic group bonded directly or through another atomic group to the surface thereof; and forming the color image by using a color ink containing an anionic substance and having a buffering capability against a pH change.